PacNOG 6: Nadi, Fiji

IP Basics

Steven VanDevender Hervey Allen

Network Startup Resource Center

Complex problems can be solved using the common divide and conquer principle. In this case the internals of the Internet are divided into separate layers.

–  Makes it easier to understand –  Developments in one layer need not require changes in

another layer –  Easy formation (and quick testing of conformation to)

standards

Two main models of layers are used: –  OSI (Open Systems Interconnection) –  TCP/IP

Layers

OSI Model

Conceptual model composed of seven layers, developed by the International Organization for Standardization (ISO) in 1984. Layer 7 – Application (servers and clients etc web browsers, httpd) Layer 6 – Presentation (file formats e.g pdf, ASCII, jpeg etc) Layer 5 – Session (conversation initialisation, termination, ) Layer 4 – Transport (inter host comm – error correction, QOS) Layer 3 – Network (routing – path determination, IP[x] addresses etc) Layer 2 – Data link (switching – media acces, MAC addresses etc) Layer 1 – Physical (signalling – representation of binary digits)

Acronym: All People Seem To Need Data Processing

OSI

Generally, TCP/IP (Transmission Control Protocol/Internet Protocol) is described using three to five functional layers. We have chosen the common DoD reference model, which is also known as the Internet Reference Model.

–  Process/Application Layer consists of applications and processes that use the network.

–  Host-to-host transport layer provides end-to-end data delivery services.

–  Internetwork layer defines the datagram and handles routing of data. –  Network access layer consists of routines for accessing physical

networks.

(See http://en.wikipedia.org/wiki/

TCP/IP

Browser MUA

HTTP SMTP

TCP UDP

DNS RTSP

Video Player

ICMP

PING

IP

802.11 WiFi Ethernet PPP

Copper Fiber Pigeons Air :)

TCP/IP model – the “hourglass”

Notice that we do not really have clear 3, 4 or 5 layers here … :)

OSI and TCP/IP

Lower layers add headers (and sometimes trailers) to upper layers packets

Application

Transport

Network

Data Link

Data

Data Header

Transport Packet Header

Data Header Header

Network Packet

Data Header Header

Header

Header

Trailer

Trailer

Encapsulation & Decapsulation

Different names for packets at different layers:

–  Ethernet (link layer) frame –  IP (network layer) datagram –  TCP (transport layer) segment

Terminology is not strictly followed we often just use the term “packet” at any layer

Frame, Datagram, Segment, Packet

Networking is a problem approached in layers. OSI Layers TCP/IP Layers

Each layer adds headers to the packet of the previous layer as the data leaves the machine (encapsulation) and the reverse occurs on the receiving host (decapsulation)

Summary

32 bit number (4 octet number) can be represented in lots of ways:

133 27 162 125

10000101 00011011 10100010 01111101

85 1B A2 7D

So What is an IPv4 Address Anyway?

Decimal

Binary

Hexadecimal

Network Masks help define which bits are used to describe the Network Part and which for hosts

Different Representations: –  decimal dot notation: 255.255.224.0 (128+64+32 in byte 3) –  binary: 11111111 11111111 111 00000 00000000 –  hexadecimal: 0xFFFFE000 –  number of network bits: /19 (8 + 8 + 3)

Binary AND of 32 bit IP address with 32 bit netmask yields network part of address

Network Masks

Hierarchical Division in IP Address: Network Part (Prefix)

Describes which network

Host Part (Host Address) Describes which host on that network

Boundary can be anywhere Used to be a multiple of 8 (/8, /16/, /24), but not usual today

Network Host

205 . 154 . 8 1

11001101 10011010 00001000 00000001 Mask

More to the Structure

137.158.128.0/17 (netmask 255.255.128.0)

1000 1001 1001 1110 1 000 0000 0000 0000

1111 1111 1111 1111 1 000 0000 0000 0000

1100 0110 1000 0110 0000 0000 0000 0000

1111 1111 1111 1111 0000 0000 0000 0000

1100 1101 0010 0101 1100 0001 10 00 0000

1111 1111 1111 1111 1111 1111 11 00 0000

198.134.0.0/16 (netmask 255.255.0.0)

205.37.193.128/26 (netmask 255.255.255.192)

Sample Netmasks

The subnet mask is used to define size of a network E.g a subnet mask of 255.255.255.0 or /24 implies 32-24=8 host bits

–  2^8 minus 2 = 254 possible hosts

Similarly a subnet mask of 255.255.255.224 or /27 implies 32-27=5 host bits

–  2^5 minus 2 = 30 possible hosts

Allocating IP Addresses

All 0’s in host part: Represents Network –  e.g. 193.0.0.0/24 –  e.g. 138.37.128.0/17 –  e.g. 192.168.2.128/25 (Why?)

All 1’s in host part: Broadcast (all hosts on net) –  e.g. 137.156.255.255 (137.156.0.0/16) –  e.g. 134.132.100.255 (134.132.100.0/24) –  e.g. 192.168.2.127/25 (192.168.2.0/25) (Why?)

127.0.0.0/8: Loopback address (127.0.0.1) 0.0.0.0: Various special purposes (DHCP, etc.)

Special IP Addresses

/24

/25

/27

....

By adding one bit to the netmask, we subdivide the network into two smaller networks. This is subnetting.

i.e.: If one has a /26 network (32 – 26 = 6 => 2^6 => 64 addresses), that network can be subdivided into two subnets, using a /27 netmask, where the state of the last bit will determine which network we are addressing (32 – 27 = 5 => 2^5 => 32 addresses). This can be done recursively (/27 => 2 x /28 or 4 x /29, etc...).

Example: 192.168.10.0/25 (.0 - .127) can be subnetted into 192.168.10.0 / 26 and 192.168.10.64 / 26

/27

/27

/27 /27

/27

/27

/27

/26

/26

/26

/26

/25

Networks – Super- and Subnetting

/24

/25

/25

Inversely, if two networks can be “joined” together under the same netmask, which encompasses both networks, then we are supernetting.

Example:

Networks 10.254.4.0/24 and 10.254.5.0/24 can be “joined” together into one network expressed: 10.254.4.0/23.

Note: for this to be possible, the networks must be contiguous, i.e. it is not possible to supernet 10.254.5.0/24 and 10.254.6.0/24

/26

/26

/26

/26

Networks – Super- and Subnetting

Private IP address ranges (RFC 1918) –  10/8 (10.0.0.0 – 10.255.255.255) –  192.168/16 (192.168.0.0 – 192.168.255.255) –  172.16/12 (172.16.0.0 – 172.31.255.255)

Public Address space available from your Regional Internet Registry Choose a small block from whatever range you have, and subnet your networks (to avoid problems with broadcasts, and implement segmentation policies – DMZ, internal, etc...)

Numbering Rules

RIRs –  AfriNIC

Africa –  APNIC

Asia and Pacific –  ARIN

United States, Canada and parts of Caribbean –  LACNIC

Latin America and Caribbean –  RIPE NCC

Europe, Middle East and Central Asia

There is overlap and much history behind these registries.

Regional Internet Registries

Files (Debian/Ubuntu)

/etc/network/interfaces /etc/hosts /etc/hostname (optional)

Commands

–  ifconfig eth0 192.168.100.x/24 –  route add default gw 192.168.100.1 –  hostname pc1.workshop.domain

Some Linux IP-related Settings

Every host on the internet needs a way to get packets to other hosts outside its local network.

This requires special hosts called routers that can move packets between networks.

Packets may pass through many routers before they reach their destinations.

Routing

All hosts (including routers) have a route table that specifies which networks it is connected to, and how to forward packets to a gateway router that can talk to other networks. A simple route table (as might be shown by route -n): Destination Gateway Genmask Flags Interface 128.223.142.0 0.0.0.0 255.255.254.0 U eth0 0.0.0.0 128.223.142.1 0.0.0.0 UG eth0

The Route Table

Destination Gateway Genmask Flags Interface 128.223.142.0 0.0.0.0 255.255.254.0 U eth0 0.0.0.0 128.223.142.1 0.0.0.0 UG eth0

The Destination is a network address. The Gateway is an IP address of a router that can forward packets (or 0.0.0.0, if the packet doesn't need to be forwarded). The Genmask is a netmask. Flags indicate the type of route. The Interface is the network interface that is connected to that network.

What do Route Table Entries Mean?

A packet that needs to be sent has a destination IP address.

For each entry in the route table (starting with the first):

1.  Compute the logical AND of the destination IP and the Genmask entry.

2.  Compare that with the Gestination entry. 3.  If those match, send the packet out the Interface, and we're

done. 4.  If not, move on to the next entry in the table.

How the Route Table is Used

Suppose we want to send a packet to using this route table. Destination Gateway Genmask Flags Interface 128.223.142.0 0.0.0.0 255.255.254.0 U eth0 0.0128.223.143.42.0.0 128.223.142.1 0.0.0.0 UG eth0

In the first entry: 128.223.143.42 AND 255.255.254.0 = 128.223.142.0 This matches the Destination of the first routing table entry, so send the packet out Interface eth0.

The first entry is called a network route.

Reaching the Local Network

Suppose we want to send a packet to 72.14.213.99 using this route table?

Destination Gateway Genmask Flags Interface 128.223.142.0 0.0.0.0 255.255.254.0 U eth0 0.0.0.0 128.223.142.1 0.0.0.0 UG eth0

1.  72.14.213.99 AND 255.255.254.0 = 72.14.212.0 2.  This does not match the first entry, so move on to the next

entry. 3.  72.14.213.99 AND 0.0.0.0 = 0.0.0.0 4.  This does match the second entry, so forward the packet to

128.223.142.1 via eth0.

Reaching Other Networks

Note that this route table entry: Destination Gateway Genmask Flags Interface 0.0.0.0 128.223.142.1 0.0.0.0 UG eth0

…matches every possible destination IP address. This is called the default route.

The gateway device (128.223.142.1) has to be a router capable of forwarding traffic.

The Default Route

Consider this route table: Destination Gateway Genmask Flags Interface 192.168.0.0 0.0.0.0 255.255.255.0 U eth0 192.168.1.0 0.0.0.0 255.255.255.0 U eth1 192.168.2.0 0.0.0.0 255.255.254.0 U eth2 192.168.4.0 0.0.0.0 255.255.252.0 U eth3 0.0.0.0 192.168.1.1 0.0.0.0 UG eth0

This is what a router's routing table might look like. Note that there are multiple interfaces for multiple local networks, and a gateway that can reach other networks.

More Complex Routing

Any UNIX-like (and other) operating system can function as gateway:

In Linux in /etc/sysctl.conf set:

net.ipv4.ip_forward=1

Without forwarding enabled, the box will not forward packets from one interface to another: it is simply a host with multiple interfaces.

Forwarding Packets